Slits

The available slits are shown in the following table. The slit widths are given in pixels and arcsec projected on the sky. The spectral resolution is lower in a wider slit (see the grism table), even for point sources since under most atmospheric conditions the seeing wings of pointlike objects fill the wider slits. Note that for each f/6 slit width, 2 slits are available, one of which is offset from the optical axis and causes the spectrum to be shifted to shorter wavelengths on the array. Note also that the f/32 10 pixel wide slit and the f/6 2 pixel wide slit are the same slit.

Slit name (arcsec)

Gemini ID

Slit width (pixels)

Slit width (arcsec)

Slit length (arcsec)

f/6 2-pixel centered

G5211

1.94

0.226

50

f/6 4-pixel centered

G5212

4.02

0.470

110

f/6 6-pixel centered

G5213

6.42

0.750

110

f/6 2-pixel blue

G5214

1.94

0.226

50

f/6 4-pixel blue

G5215

3.5

0.409

50

f/6 6-pixel blue

G5216

6.0

0.696

50

f/32 4-pixel centered*

G5229

3.2-4.4*

0.07-0.10

22

f/32 7-pixel centered

G5230

6.6

0.144

22

f/32 10-pixel centered**

G5211

10

0.22

22

* Width varies monotonically across slit, narrowest width at top.
** This is the f/6 2-pixel wide slit used with the f/32 camera.

Slit throughputs for the three different f/6 slit widths are presented in the following
table as a function of image quality. Note that these throughputs are for an S/N-optimized
software aperture of 1.4 times the image FWHM and are not based on the total signal within
the slit. The shape of the model tip-tilt corrected PSF does not vary signficantly across
the 1-5um range and so this table is independent of wavelength. (Of course the delivered
image quality does vary with wavelength and is described as part of the observing condition constraints).
The values in this table were derived using the NIRI Integration Time Calculator.

Slit width (arcsec)

Throughput as function of
image quality (50% EED in arcsec from 0.3-0.9 arcsec)

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.232

0.58

0.45

0.36

0.30

0.26

0.23

0.19

0.464

0.86

0.73

0.64

0.55

0.49

0.45

0.40

0.696

0.87

0.83

0.80

0.71

0.65

0.62

0.56

Slitless spectroscopy, sometimes used for flux calibration instead
of or in addition to imaging, is available with NIRI.
The focal plane wheel containing the slits is controlled independently
of the wheel containing the grisms. One can therefore image a source
with or without a grism and with or without a slit. Sensitivities for slitless
spectroscopy are much lower than when slits are used, and thus the technique
is not useful on very faint sources.

Positioning of the slits and grisms appears to be repeatable at the pixel level, but
are not perfectly repeatable at the sub-pixel level.
During long integrations (>30 minutes) it is necessary to move the grism (and possibly
the slit as well) out of beam to check the pointing. This may mean that the
wavelength coverages before and after reacquiring the target are not precisely the same.